The present invention relates generally to electronic ballasts for powering gas discharge lamps.
More particularly, this invention pertains to circuits and methods using in an electronic ballast for detecting a lamp end of life condition and/or a short-circuit fault condition at the ballast output.
For safety and equipment reliability purposes, electronic ballasts used in fluorescent lighting must include protection circuitry for lamp end of life (EOL) conditions. This need is particularly significant for T5 or smaller lamps. Preferably, the EOL protection circuit will shut down the ballast when the lamp reaches an EOL condition.
A typical class D inverter topology for an electronic ballast is shown in FIG. 1. A DC rail voltage V_rail is conventionally outputted by a voltage source such as a power factor correction (PFC) section (not shown) or a rectifier circuit (not shown). The rail voltage V_rail is converted by a half-bridge inverter into a high frequency AC voltage. In the embodiment of FIG. 1, switching elements Q1 and Q2 are MOSFETs that are driven by an IC driver circuit. Capacitor C_dc_blocking is a DC blocking capacitor which prevents DC current from going through the resonant inverter output circuit defined by resonant inductor T_resonant and resonant capacitor C_resonant. A gas discharge lamp (Lamp) is connected across the resonant capacitor C_resonant. The resonant circuit provides proper lamp starting and steady state voltages for the Lamp. Capacitor C_lamp_block is also a DC blocking capacitor to prevent any DC current from passing through the lamp in the output.
When a fluorescent lamp reaches its end of life, the lamp voltage typically pulses asymmetrically and the lamp may exhibit visible flickering. The asymmetric pulse will generate a DC voltage offset across the lamp.
What is needed is a lamp EOL protection circuit for an electronic ballast that can exploit the existence of the asymmetric lamp voltage pulses to sense that the lamp is in an end of life condition and then initiate appropriate actions to protect the ballast.